Electrical stimulation of nerve trunks and their branches is known to be effective in the treatment of a variety of neurological disorders in humans spanning from treatment of incontinence to gait disorders. Sensing and recording nerve signals is a discipline that aims for obtaining valuable input for actively controlling the timing of the electrical stimulation of nerves. The recorded nerve signals can also be used for controlling equipment placed outside the body as e.g. prostheses that serve as functional replacement of body parts.
When it comes to the art of electrical stimulation of nerves for the treatment of gait disorders, especially correcting drop-foot, electrodes are placed in the proximity of the peroneal nerve or its branches. An implantable pulse generator connected to the electrode generates a pattern of pulses to stimulate the nerve which will cause the foot dorsiflexor muscles to contract. Thus the foot will be lifted and it will be possible for the patient to swing the leg more naturally while walking. An example of a system for correction of drop-foot is known from EP 1 257 318 B1 to Neurodan A/S. The document covers the medical aspects and discloses examples of various preferred embodiments. For the triggering of the electrical stimulation of the nerve, according to the wanted reaction of the foot, the use of a heel switch is disclosed. The heel switch can be either connected to the pulse generator with electrical wires or it can include a wireless transmitter module for triggering the pulse generator. For the interface between the pulse generator and the electrode the system comprises an inductive link, an antenna to be mounted on the skin of the patient and a counterpart in form of an implantable antenna adapted to be implanted in the thigh of the patient. In a further embodiment it is shown that neural information recorded on e.g. the Sural nerve can be used for determining certain gait events such as heel strike and heel lift. For recording the neural information a nerve recording electrode is used, the nerve recording electrode in the preferred embodiment being a CUFF electrode, in principle a tube of insulating material with a number of contacts placed on the inside of the tube. The CUFF electrode is in one embodiment a multipolar nerve stimulation and recording electrode where the electrode is switched between a mode of recording nerve signals and a mode where electrical nerve stimulation is carried out. As can be seen in FIG. 1, natural sensors can be used as trigger input for a drop foot stimulator. Gait related information can be either sensed from a dedicated sensing electrode on a purely sensory nerve, or through the same electrode that the mixed common peroneal nerve (sensory and motor branches) is being stimulated with.
When it comes to recording information from natural sensors in living beings, information is encoded as action potentials, which are propagating along nerve fibers, either from their natural sensors, or to their muscles. An action potential is a transient change in the voltage between the intracellular (within the nerve fiber) and extracellular space (outside the nerve fiber) on either side of the membrane, as result of a mechanical, electrical or chemical stimulus that changes the electrochemical balance. This local disturbance can cause imbalance in the neighboring nerve tissue, allowing the action potential to propagate along the nerve. As a result of the short lasting disturbance at any given point on the nerve, ionic currents are flowing into and out of the membrane of the nerve cells. It is these membrane action currents, which allow the pickup of nerve activity with electrodes adjacent to the nerve, so-called extracellular electrodes.
If an electrode is placed on a cut nerve ending where the intracellular fluid makes good contact with restricted extracellular field, and a second electrode is placed further along the uninjured nerve, the shape of the extracellularly recorded action potential is identical to that of the membrane action potential at the second electrode [R. B. Stein and K. G. Pearson. amplitude and form of action potentials recorded from unmyelinated nerve fibres. J. Theoretical biology 32:539-558, 1971]. FIG. 2, shows the setup for a monopolar recording with an electrode placed around the nerve. The reference electrode is arranged far away from the recording electrode. Whenever the action potentials propagates under the electrode, the associated action currents causes voltage drops that can be picked up by the extracellular electrode. The voltage waveform approaches a scaled version of the action potential, with a scaling factor that depends on the transverse and longitudinal conductivity of the medium surrounding the nerve.
The monopolar configuration has the disadvantage that other biological interference as for instance caused by adjacent muscle activity will be indistinguishably picked up between recording and reference electrode. This situation can be greatly improved by recording nerve activity between two adjacent electrodes with an instrumentation amplifier which can greatly reduce any common mode interference as shown in FIG. 3. If the electrodes are aligned parallel to the gradient of the electric interference field, a tiny fraction of the greatly extended biological interference field can be sampled as differential voltage, which is increasing with the inter-electrode distance. But the inter-electrode distance cannot be made arbitrary small, because the wavelength of the action potentials increases with the nerve conduction velocity, and thus requires a larger inter-electrode distance for proper spatial sampling especially for fast conducting nerve fibers.
As previously mentioned, the amplitude of the action potentials recorded with an extracellular electrode is also dependent on the conductivity of the surrounding medium. It was found that the amplitude was proportional to the ratio between extracellular and axioplasmatic (i.e. the ohm'ic resistance inside of the nerve) resistivity [A. L. Hodgkin and W. A. Rushton. The electrical constants of a crustacean nerve fibre. Proc. R. Soc. Med. 134 (873):444-479, 1946].
Researchers have shown that if a nerve is brought into another electrically isolating medium like air (lifted the nerve with the attached hook electrode from the biological medium) or paraffin, the voltages significantly increase [L. Hermann. Untersuchungen ueber die Aktionsstroeme des Nerven: Teil II. Pfluger's Arch. ges. Physiol. 24:246-294, 1881], [K. S. Cole and H. J. Curtis. Membrane Potential of the Squid Giant Axon during current flow. J. Gen. Physiol. 24 (4):551-563, 1941]. This led researchers to the idea of surrounding the recording electrodes by an insulating silastic nerve cuff [R. B. Stein, D. Charles, L. Davis, J. Jhamandas, A. Mannard, and T. R. Nichols. Principles underlying new methods for chronic neural recording. Canadian Journal of Neurological Sciences:235-244, 1975], [J. A. Hoffer and G. E. Loeb. Implantable electrical and mechanical interfaces with nerve and muscle. Ann. Biomed. Eng 8:351-369, 1980].
These cuff electrodes can be produced by molding the electrode into silastic sheets that are wrapped around the nerve, and closed by a suture. As the silicone cuff is surrounding the recording electrodes, it also reduces the interference voltages between the two recording electrodes.
The interference can be further reduced by recording from a center electrode inside the cuff, against two short-circuited end electrodes [R. B. Stein, D. Charles, L. Davis, J. Jhamandas, A. Mannard, and T. R. Nichols. Principles underlying new methods for chronic neural recording. Canadian Journal of Neurological Sciences: 235-244, 1975]. This configuration is herein called quasi-tripolar, and shown in FIG. 4, where all previously mentioned recording configurations are depicted within a single sealed cuff electrode. The gradient of the electric field parallel to the nerve axis is slowly and monotonically changing. The connected end-electrodes measure the average of the distant source field at the ends of the recording zone, which is an estimate of the distance source field at the center electrode. Near field sources, such as action potentials within the nerve, are not monotonically changing, and thus the two end electrodes do not estimate the action potential at the center electrode. Thus, potential differences in the near field become differential mode, while those from distant sources become common mode. However, a residual differential interference cannot be avoided, due to the unobtainable perfect matching of all electrode- and inter-electrode impedances that would be required.
FIG. 5, shows that a bridge circuit may be chosen as a topology to describe the quasi-tripolar configuration when exposed to interference [M. Rahal, J. Winter, J. Taylor, and N. Donaldson. An improved configuration for the reduction of EMG in electrode cuff recordings: a theoretical approach. Biomedical Engineering, IEEE Transactions on 47 (9):1281-1284, 2000]. The model suggests that the output voltage is a product of both bridge voltage and a function of the mismatch between the tissue impedances between the electrodes (Rt1, Rt2) as well as the electrode-tissue impedances of the end electrodes Ze1 and Ze3. The later determine the amount of current that is flowing through the bridge wire, which always will be smaller than the total interference current flowing into the cuff. By bridge wire we refer to the (ideally) conducting wire that is connected between the end electrodes Ze1 and Ze3. If the end electrodes were perfect conductors with Ze1=Ze3=0 Ohm, all the interference current would flow through the bridge wire, and the bridge voltage would be zero. In that case, the mismatch of the impedances would not matter at all, and no interference voltage would appear at the output.
Unfortunately not all the interference current can be by-passed, due to the electrochemical properties of the electrode. The reactive parts of the electrode impedance can be decreased by proper surface treatment, but the access impedance is determined by the geometry of the contact disks as well as the amount of proliferated scar tissue encapsulating the contacts. Attempts were made to decrease the impedances of the end electrodes by adding additional electrodes, shorted together, as shown in FIG. 6. [L. N. S. Andreasen and J. J. Struijk. Artefact reduction with alternative cuff configurations 22. Biomedical Engineering, IEEE Transactions on 50 (10):1160-1166, 2003]. However, in comparison to the standard quasi-tripolar configuration, an improvement in the range between 18% and 24% was attributed to the decrease in end-electrode impedance.
The above described research overview is the basis for the design of the system granted in EP 1 257 318 B1 to Neurodan A/S which has been drawn up in the preamble of this application. The electrode design of the system works well both for sensing and stimulation. However, when it comes to sensing, a better separation of the signal from signals originating from biological interference sources as e.g. muscles would be appreciated.